Method of producing plasma sprayed titanium carbide tool steel coatings

ABSTRACT

A method is disclosed for quench-depositing by plasma spraying onto the substrate an adherent thin coating of a titanium carbide tool steel containing by weight about 10% to 80% of primary grains of TiC and the balance essentially about 90% to 20% of a steel matrix, such that the resulting coating is one characterized metallographically by the presence of martensite. The metal substrate may be aluminum. Preferably, the titanium carbide grains should be rounded to assure a low friction surface.

I United States Patent 1 91 on 3,896,244

Ellis et al. 1 July 22, 1975 METHOD OF PRODUCING PLASMA 3.368.882 2/1968Ellis et al. 1 29/195 A SPRAYED TITANIUM CARBIDE TOOL 3,416,976 12/1968Brill-Edwards 148/126 X 3,440,079 4/1969 Jensen 117/22 STEEL COATINGS3,539,192 11/1970 Prasse 117/93,] PF X 175] inventors: John L. Ellis,White Plains; M. 3,674,544 7/1972 Grosseau 117/1052 X Kumar MM, Springvalley; Swan 3,787,229 1/1974 Rudness 117/9 x E. Tarkan, MOnSCy, all Of[73] Assignee: Chromalloy American Corporation, Elfimems f p mMetallurgy Mben Guy, pages New York 489-492, Addison-Wesley PublishingCompany, Inc.

[221 Filed; June 8, 1973 Primary ExaminerWi1liam D. Martin [2| 1 App.368063 Assistant ExaminerStuart D. Frenkel Related US. Application DataAttorney, Agent, or Firm-Hopgood, Calimafde, Kalil [62] Division of Ser.No, 199,497, Nov. 17, 1971, Pat No.

3,779,720. [57] ABSTRACT [52] U 5 Cl 427/34 427/383 427/423 A method isdisclosed for quench-depositing by 1511 Int Cl llllllllllllllll u BOSD"/10 plasma spraying onto the substrate an adherent thin coating of atitanium carbide tool steel containing by [58] Field Search Z Z Q g Egiweight about 10% to 80% of primary grains of TiC and the balanceessentially about 90% to 20% of a steel matrix, such that the resultingcoating is one References Cited characterized metallographically by thepresence of UNlTED STATES PATENTS martensite, The metal substrate may bealuminum 2.998.322 8/1 61 tratc 117/22 Preferably, the titanium carbidegrains should be rounded to assure a low friction surface. 6 owa aym.3,355,264 11/1967 Kempe 29/195 A 7 Claims, 2 Drawing Figures W L i a aMETHOD OF PRODUCING PLASMA SPRAYED TITANIUM CARBIDE TOOL STEEL COATINGSThis application is a division ofcopcnding US. application Scr. No.199.497. filed Nov. 17. l)7l.now USv Pat. No. 3.779.720.

This invention relates to a method for producing an adherent. hard, wearresistant coating of a heat treatable titanium carbide tool steel onto ametal substrate (cg. steel) and. in particular. to a method for producing such coatings on relatively soft metal substrates. such as aluminum.copper. silver and the like. whereby the hard titanium carbide steelcoating can be further heat treated. if desired. at temperatures belowthe melt ing point of the substrate metal. The invention also relates tocomposite metal structures produced by the method.

STATE OF THE ART It is known to hard face metal substrates by usingwelding and brazing methods in which the metal substrate issimultaneously heated during the laying down of the hard facingmaterial. Because of the general nature of the foregoing process. themetal substrates were limited to those having fairly high meltingpoints. otherwise, the substrate would overheat and either melt or beadversely affected.

One attempt to enlarge the use of hard facing has been to employ flamespraying. This method comprises melting powder metal compositions in aheated zone and propelling the molten particles to the surface of ametal substrate to form a coating thereon. This method had itslimitations as to the type of materials that could be sprayed. Forexample. if refractory carbide particles are sprayed. generally a matrixmetal powder is mixed with it. e.g.. nickel. cobalt. etc., and themixture sprayed to provide the means by which to anchor the carbideparticles to the receiving surface. So long as no further heat treatmentis required of the coating. certain types of hard coatings could beproduced. although they tended to be porous.

In recent years. a special kind of hard titanium carbide tool steel hasbeen developed which. besides having the intrinsic hardness of thetitanium carbide, also is capable of being further hardened very much astool steel is hardened. For example. a titanium carbide tool steelcontaining 33% by weight of TiC (about 45% by volume) and the balance achromium-molybdenum steel (note US. Pat. Nos. 2.828.202 and 3.4l6.976)requires a relatively high temperature for heat treatment. Thus. toobtain a martensitic matrix. the titanium carbide tool steel compositionis quenched from about l.750F in oil. However. the foregoing heattreating temperature is higher than the melting point of certain metalsubstrates. such as aluminum. Moreover. the conventionally sprayedcoatings tend to be quite porous.

Hard carbide coatings would be desirable on certain substrate metals.such as metals having relatively high thermal and electricalconductivity. for example. alu' minum, copper. silver and the like. Itwould be desirable to provide such coatings having minimum porosity andexceptionally good wear resistance. Such coatings would be useful inproviding long life electric contact metal having a hard. wear resistantcontact face and a substrate of good thermal and electricalconductivity. Such coatings would also be useful in producing aluminumelements. eg. a housing for the recently developed rotary combustionengine. having a hard surface coating to provide resistance to wearrelative to the ro tary piston in contact therewith.

OBJECTS OF THE INVENTION It is thus the object of the invention toprovide a method of producing a hard. dense wear resistant coat ing ofatitanium carbide tool steel on a metal substrate.

Another object is to provide a method for producing a composite articleof manufacture comprising a metal substrate having an adherent densecoating of a titanium carbide tool steel comprised metallographically ofprimary grains of titanium carbide dispersed substantially uniformlythrough a steel matrix characterized by an austenitic decompositionproduct comprising martensite. Other hard phases may be present, such asbainite. and the term martensite" employed herein is meant to covermartensite with or without retained austenite and mixtures of martensitewith bainite. with or without retained austenite.

An additional object is to provide a method for pro ducing a titaniumcarbide tool steel coating characterized metallographically by roundedprimary grains of titanium carbide, for example. on metal substrates ofmelting point above l.l00F.

A still further object of the invention is to provide a method forforming a titanium carbide tool steel hardfacing coating on a metalsubstrate of aluminous metal in which the coating is comprisedmetallographically by primary grains of titanium carbide dispersedsubstantially uniformly through a steel matrix characterized by anaustenitic decomposition product containing martensite.

These and other objects will more clearly appear when taken intoconsideration with the following disclosure and the accompanyingdrawing. wherein:

FIG. 1 depicts schematically a device for the plasma flame spraying ofmetal powder; and

FIG. 2 shows schematically a rotary combustion engine utilizing a heattreatable. titanium carbide steel as a hard-facing material on the innerend walls thereof.

STATEMENT OF THE INVENTION Stating it broadly. the method aspect of theinvention for producing a wear resistant coating of a heat treatabletitanium carbide tool steel on a metal substrate resides in selecting apowder composition consisting essentially of about 10 to by weight ofprimary grains of titanium carbide and the balance essentially to 20% byweight of steel-forming ingredients and quench-depositing saidcomposition from the molten state onto a metal substrate by means ofaplasma flame which heats the steel ingredients to substantially abovethe melting point. whereby a dense, adherent coating of the compositionis produced on the metal substrate. the coating being thin relative tothe metal substrate and preferably ranging up to about 0.025 inch inthickness.

By employing the plasma flame for depositing the coating. rather hightemperatures are obtained which melt the steel matrix of the compositionat temperatures substantially above the melting point. such that thincoatings deposited on the metal substrate are drastically quenched byvirtue of the cooling effect of the substrate to produce amicrostructure comprising grains of titanium carbide dispersed through amatrix formed of an austenitic decomposition product containingmartensite. The metal substrate should preferably have a melting pointabove I 100F.

As stated hereinabove. very high temperatures are obtainable with theplasma flame. However. for most spray applications. a plasma temperatureof about l2.000 to 20.000F appears to be optimum. One of the advantagesof the plasma flame is that it can be used with a controlled atmosphere.This is important to avoid decarburization of the steel matrix wherecarbon is essential to the heat treatment response to the titaniumcarbide tool steel. Thus. an inert gas or a chemically inactive gas canbe employed for the flame medium.

The plasma flame is produced by striking an arc between a cathode and ananode and passing a plasma gas through the arc. By confining the arc ina chamber under pressure. the arc temperature can be increased. Byconstructing the anode as a hollow nozzle and introducing the plasma gasinto the arc chamber and forcing it through the nozzle. the gasdissociates and ionizes in the arc stream and emerges from the nozzle asa plasma flame. A typical plasma gas is one comprised of 90% nitrogenand l% hydrogen. Argon or other gases can be used in place of nitrogen.

A schematic representation of a plasma flame device is given in theaccompanying drawing which shows cathode l0 and anode ll electricallyconnected via power source 12 to produce an arc stream 13. Plasma gas14. e.g.. 90% nitrogen and l0% hydrogen. is fed through pipe 15 which isconverted to plasma 16 which exits through nozzle 17 at a very hightemperature as free plasma. Spray powder is fed through pipe [9 into thenozzle where it is heated by the plasma flame and exits with the freeplasma towards the workpiece or substrate to be coated. Plasma guns formetal powder spraying are readily available and therefore need not bediscussed any further than the schematic described above.

One of the advantages of plasma spraying is that relativcly thincoatings can be sprayed which are dense and substantially free of largepores. By spraying thick nesses ranging up to about 0.025 inch. highlyrapid quenching of the deposit is obtained generally comprised ofmartensite. Where the metal substrate is a metal of substantially highthermal conductivity. e.g.. aluminum. copper. silver and the like. veryhard coatings are obtained which can be further heat treated. e.g..tempered or aged. at temperatures below the melting point of thesubstrate. However. the coating can also be applied with advantageousresults to ferrous metal substrates. e.g.. steels.

DETAILS OF THE INVENTION As illustrative of titanium carbide tool steelcompositions that can be plasma sprayed onto metal substrates. thefollowing examples are given.

EXAMPLE I Broadly. the titanium carbide tool steel is comprisedessentially of about 10 to 80% by weight of primary grains of titaniumcarbide dispersed through a steel matrix making up about 90% to 20% ofthe balance. The steel may be low and high carbon steel. medium alloysteel or high alloy steel containing at least 50% iron which. whencooled substantially rapidly from above the melting point. providesmetallographically a matrix containing an austenitic decompositionproduct containing martensite. In this connection. reference is made toUS. Pat. No. 2.828.202. Examples ofsuch ma trix steels are: SAE 1010 toSAE l080 steels. and in cluding the following illustrative composition.to wit: 0.8% Cr. 0.2% Mo. 0.3% C and iron substantially the balance; 5%Cr. l.4% Mo. 1.4% W. 0.45% V. 0.35% C and iron substantially thebalance; 8% Mo. 4% Cr. 2% V. 0.8% C and iron substantially the balance;18% W. 4% Cr. 1% V. 0.75% C and iron substantially the balance; 20% W.l2% Co. 4% Cr. 2% V. 0.8% C and iron substantially the balance.

A preferred composition is one containing about I to 6% Cr. about 0.3 to6% Mo. about 0.3 to 0.8% C and the balance essentially iron.

EXAMPLE 2 A particular titanium carbide tool steel is one containing l0to by weight of TiC and the balance essentially a high chromium highcarbon steel containing about 6 or 7 to l2% chromium. 0.6 to l.2%carbon. 05 to 5% molybdenum. up to about 5% tungsten. up to about 2%vanadium. up to about 3% nickel. up to about 5% cobalt. and the balanceessentially iron. A preferred composition of the foregoing highchromium. high car bon steel is one one containing about l0% chromium.l% carbon. 3% molybdenum. l% vanadium. and the balance essentially iron.This steel is characterized in that it forms martensite when appliedfrom a plasma spray onto a relatively cold substrate. e.g.. steel.aluminum. and the like and. by double tempering at 1.000F for 1 houreach. the hardness is further augmented by secondary hardening. while.at the same time. the coating is substantially stress relieved of anythermal stresses due to the rapid cooling when deposited. As will benoted. the tempering temperature l.00OF) is below the melting point ofaluminum.

EXAMPLE 3 As illustrative of another titanium carbide tool steelcomposition that can be plasma sprayed onto a metal substrate and whichcan be further heat treated at a temperature below the melting point ofthe substrate is a heat treatable. low carbon nickel-containing titaniumcarbide tool steel (note US. Pat. No. 3.369.89l As in the foregoingexamples. the titanium carbide ranges from about It) to 80% by weightand the steel matrix from about to 20% by weight. The matrix compositioncontains by weight about [0 to 30% nickel. 0.2 to 9% of titanium. and upto about 5% aluminum. the sum of the titanium and aluminum not exceedingabout 9%. up to about 25% cobalt. up to about l0% molybdenum. andsubstantially the balance of the matrix at least about 50% iron; themetals making up the matrix composition being proportioned such thatwhen the nickel content ranges from about ID to 22% and the sum of thealuminum and titanium is less than 1.5%. the cobalt and molybdenumcontents are each at least about 2%; and such that when the nickelcontent ranges from about 18 to 30% and the molybdenum content is lessthan 2%. the sum of the aluminum and titanium exceeds 1.5%.

When the foregoing titanium carbide tool steel is deposited from theplasma flame and rapidly quenched. the metallographic structure isessentially soft martens ite. In this condition. the carbide steel inthe form of a coating can be age-hardened by heating it to a temperatureof about 500F to l.200F (260C to 650C) for about 3 hours. A typicalage-hardening temperature is 900F (483C). As will be noted. theage-hardening temperature is below the melting point of aluminum.Normally. the solution temperature for obtaining soft martensite in thematrix ranges from about l.400F to 2.l50F (760C to l.l65C). As will beobserved. the foregoing temperature range is above the melting point ofaluminum. However. a solution treatment is not necessary for the coatingsince the steel is solutionquenched due to the rapid cooling followingplasma spraying of the composition.

A typical composition is one containing about 35% by weight of TiC andthe 65% remainder a steel matrix containing 21.7% Ni, 8.49% Co, 3.42%Mo. 0.37% Ti and the balance essentially iron. The alloy upon aging at900F (483C) for 3 hours exhibited a hardness of about 60 R,-.

It is preferred when working with compositions of the type illustratedin Examples 1. 2 and 3 to work with a pro-alloyed titanium carbide toolsteel. This assures the presence of rounded grains of titanium carbidewhich not only provides resistance to wear but also imparts very lowcoefficient of friction. This is important in applications involving thecontinuous rubbing of parts, such as occurs in the rotary combustionengine where the apices of the rotary piston are in continuous contactwith the inner side walls of the housing. Where the housing is made ofaluminum. a face coating of a titanium carbide tool steel of the typeillustrated in Example 2 provides adequate resistance to wear and inaddition low coefficient of friction by virtue of the rounded grains ofTiC.

ln assuring the rounded grain structure of titanium carbide. thetitanium carbide tool steel is prealloyed by liquid phase sintering apowder metallurgical composition of the carbide steel. The pre-alloyedcarbide steel is then ground into a particle size passing through 200mesh for use in plasma spraying.

in producing a pre-alloyed steel composition with rounded grains of TiC.the following method is employed.

A titanium carbide tool steel composition containing 33% by weight ofTiC (45% by volume) and substantially the balance a steel matrix, suchas a chromiummolybdenum steel composition. is produced by mixing 500grams TiC (of about 5 to 7 microns in size) with I000 grams ofsteel-forming ingredients in a mill half filled with stainless steelballs. To the powder mix is added 1 gram of paraffin wax for I grams ofmix. The milling is conducted for about 40 hours using hexane as avehicle. A specific steel-forming composition for the matrix is onecontaining 0.5% C. about 3% Cr, about 3% Mo and the remaindersubstantially iron. It is preferred to use carbonyl iron powder inproducing the mixture. A carbidic tool steel of the foregoing type isdisclosed in US. Pat. No. 3.4l6.976.

Following completion of the milling. the mix is removed and dried andcompacts of the desired shape pressed at about l t.s.i. and the compactsthen subjected to liquid phase sintering in vacuum at a temperature ofabout 2.640F (l.450C) for about one-half hour at a vacuum correspondingto microns or less. After completion of the sintering. the compacts arecooled and then removed from the furnace. The primary titanium carbidegrains which are angular before sintering. assume a roundedconfiguration as a result of 6 liquid phase sintering. By liquid phasesintering is meant heating the compact to above the melting point of thesteel matrix but below the melting point for titanium carbide, forexample, up to about lF C) above the melting point of the steel matrix.

Following the production of the sintered compact, the sintered compactmay be converted into chips by machining and the chips milled in a ballmill to a size passing through 200 mesh (e.g., l to 5 microns). Thepowder is cleaned and dried for use for plasma flame spraying. As statedabove, rounded titanium carbide grains are preferred in the ultimatecoating since this configuration imparts low friction characteristics tothe coating. the rounded grains being advantageous in wear application.

Wear is a combination of corrosion. erosion, abrasion. friction,sulfidation, fatigue. fretting and oxidation in which the net result issurface deterioration. An advantage of using a titanium carbide steel asa hard facing material is that it has a low density compared to otherhard-facing materials and provides good resistance to the foregoingphenomena. It has certain economic advantages since a unit weight of theforegoing hard-facing material on a volume basis covers more surfacearea of a metal substrate in comparison to the conventional hard-facingmaterials containing tungsten carbide which have a substantially higherdensity.

The rounded grain structure of titanium carbide, as stated hereinabove.is ideal because it imparts a low coefficient of friction to the coatingand also because titanium carbide has a very high intrinsic hardnessand, therefore, exhibits a very high resistance to wear. Moreover. theas-coated surfaces of this hard-facing material obtained after plasmaspraying are quite smooth. eg, about 100 to microinches rms. otherwisereferred to as rootmean square average" (square root of mean square).This is advantageous because the coated surface can be inexpensivelybuffed to provide maximum resistance to wear, resistance to galling andlow friction. The dense deposited coating can be finished to asmoothness of less than 5 microinches rms. using diamond lappingcompounds and other specialized techniques.

In plasma spraying a metal substrate, the surface thereof is degreased,cleaned and preferably grit blasted with chilled iron grit or purealuminum oxide grit to insure good adhesion of the hard-facing alloy tothe base material. The hard-facing material in the finely powered form(200+325 mesh) is fed into the stream of a superheated plasma gas. Theparticles are melted and are carried by the gas at high velocity to thesurface being plated. A coating can be built up to the desired thicknessby forming multiple layers. It is preferred that the coating be thin andpreferably range up to about 0.025 inch and, more preferably, up toabout 0.015 inch, to minimize cracking clue to cooling stresses.Compared to most conventional coatings. the titanium carbide steelcoating assured good mating compatability with metal substrates due toits low coefficient of thermal expansion. This property makes thismaterial attractive for wear resistant applications in the automotiveand aircraft industries.

As illustrative of a preferred embodiment of the invention. thefollowing example is given:

EXAMPLE 4 A pre-alloyed titanium carbide tool steel produced by liquidphase sintering was employed as the plasma spray powder the mesh size ofthe powder being in the range of about l70 to +325. the compositionconsisting essentially of about 33% by weight of TiC and 67% by weightofa steel matrix having the following composition by weight: 3% Cr. 3%Mo. 05% C and the balance essentially iron.

A plate of aluminum (AMS-4026) was employed as the metal substrate. Theplasma gas used comprised 90% nitrogen and hydrogen. The aluminumsurface was degreased and grit blasted with pure aluminum oxide (60)mesh) grit to provide a surface to promote adherence of the coating. Twocoatings of 0.007 inch were produced, one by spraying in air and theother by plasma spraying in air using an argon shield to minimizeformation of oxides in the coating.

The plasma spraying was conducted using a plasma spray gun referred toin the trade as Metco plasma flame spray system" consisting of aspecially constructed torch-type gun in which powdered coating material,suspended in a suitable carrier gas (N was fed into a chamber in whichplasma gas was excited to high temperatures by an electric arc.

Metallographic examination of each of the coatings showed theconstituents of the coating to be uniformly distributed. The coatingswere substantially free from cracks. massive porosity characteristic ofconventionally flame sprayed coatings and substantially free fromexcessive oxides. The specimen sprayed with the argon shield showed lessoxides present than the one sprayed without the shield. The coatingswere essentially free from inclusions at the coating substrate metalinterface. The interface itself appeared to be well pronounced withpractically no porosity. The overall porosity in the coating wasapproximately 8 or 9% which is considered good.

Microhardness of the thin coating was determined across its thicknessand found to range from about 650 to 770 VHN (200 gram load) whichcorresponds to a Rockwell C" hardness of about 52 to 60. This hardnessis characteristic of the presence of an austenitic decomposition productcontaining martensite. The foregoing steel composition in the annealedstate (pearlitic microstructure or spheroidized carbon) normally has ahardness in the neighborhood of 40 R,-.

Thus, the invention enables the production on an aluminous metalsubstrate with a hardened titanium carbide steel coating without thenecessity ofquench hardening the coating from an austenitizingtemperature of about 1.750F which is above the melting point of thealuminous substrate. The quenching effect achieved during the depositionof the coating provides the desirable austenitic decomposition product.

No spalling. chipping, flaking. cracking, and the like. were observed onthe coating surface and the general quality was good.

In a test to evaluate the adherence of the coating. a test panel ofaluminum (AMS-4026) of about 3 l.75 and 0.05 inches in size was plasmacoated with the same steel composition to a thickness of 0.005 inch. Thecoated panel was then subjected to a cup test in accordance with thePratt & Whitney Aircraft Materials Control Laboratory Manual (SectionE-53. 1963 Revision) using a 0.875 inch diameter ball and a die with a1.375 inch diameter opening to form a depression in the panel ofapproximately 0.300 inch. The coating did not exhibit any separationfrom the base metal indicat- 8 ing that the plasma sprayed coating hadgood bond strength.

A quench deposited coating of the same steel composition was produced onaluminum having a thickness of about 0015 inch. This coating had aquench-deposited hardness of over R. and up to about R,-. indicating thepresence of martensite in the coating. The coating was uniform. denseand crack-free. Hardened coatings of this type can be tempered attemperatures below the melting point of the metal substrate. A typicaltemperature for this steel may range from about 200F to 500F.

EXAMPLE 5 A hard-facing composition particularly resistant to softeningat elevated temperatures ranging up to about l.000F is one comprisingabout 35% by weight of TiC and the balance of a steel matrix comprisingessentially about 10%, Cr. 3% M0. 0.8% C and the balance essentiallyiron. As in Example 4, this steel composition is provided essentially inthe pre-alloyed condition to assure the presence of rounded primarygrains of TiC dispersed through the steel matrix.

A powder of the foregoing titanium carbide tool steel of 200+325 mesh isplasma sprayed onto a mild steel substrate of about one-fourth inchthick to produce a coating thickness of about 0.0] inch. the producedcoating being quench deposited by virtue of the mass of metal substratewhich rapidly cools the coating sufficiently to provide amartensitic-bearing metallographic structure. The hardness of thiscoating will generally be in the range of about 50 to 55 Rockwell C.However. this steel composition can be further hardened by utilizing itssecondary hardening characteristic by the formation of secondarycarbides by heating the coated metal substrate and the coating to aboutl.000F and holding at the temperature for about l'/2 hours. Thus.heating to l.000F will have a twofold function; (I to utilize thesecondary hardening characteristics of the titanium carbide steelcomposition. and (2) to minimize the effect of any residual thermalstresses in the coating arising from the rapid quenching of the coatingduring deposition from the plasma flame.

EXAMPLE 6 In addition to the foregoing example, some heat treatment datawere obtained on two plasmacoated aluminum substrate specimens.

Substrate A was coated with a pre-alloyed titanium carbide tool steelcontaining 35% by weight of TiC and the balance a steel matrixcontaining 3% Cr. 3% M0. 0.5% C with the remainder iron.

Substrate B was coated with a pre-alloyed titanium carbide tool steelcontaining 35% by weight of TiC and the balance 10% Cr. 3% Mo. 0.8% Cwith the remainder iron.

Hardness readings were obtained for the coatings as sprayed and for thecoatings double tempered at Jl)50F. Each was tempered for 1 hour attemperature. cooled and tempered again for one hour. The resultsobtained are given in the following table.

Aluminum Rockwell "C Substrate Condition Microhardness (Vickers)(Converted) A As sprayed 859 (Av. of 7 readings) 65.) Double Tempered788 (Av. of 7 readings) 63.5 B As sprayed 620 (Av. of readings) 565Double Tempered 1048 (Av. of 6 readings) 69.4

It will be noted that the hardness on Substrate A only dropped a fewpoints after double tempering; whereas, the hardness on Substrate Bincreased markedly by about l3 points. apparently due to secondaryharden Example 7 This hard-facing titanium carbide steel composition isadvantageous in that the coating deposited on a substrate, such asaluminum, can be hardened after deposition by heat treatment at atemperature below the melting point of aluminum or other substratemetal. A par ticular composition is one containing 30% by weight oftitanium carbide and the balance of 70% essentially a steel matrixcontaining about 21.5% Ni, 8.5% Co, 3.4% M0, 0.4% Ti and the balanceessentially iron.

The foregoing steel in the pre-alloyed condition and as a powder ofS0+325 mesh size is plasma sprayed onto aluminum as described in Example4. By using a steel matrix very low in carbon, e.g., below 0.15%, softmartensite is obtained in the coating. After the substrate has beencoated, the substrate and the coating are heated to a temperature ofabout 900F as described in Example 3 to harden the steel to the desiredhardness.

An advantage of the foregoing steel composition is that the coating canbe buffed to the desired smoothness and then age hardened at theforegoing temperature to desired hardness.

The methods described in Examples 4 to 7 can be applied to a largevariety of metal substrates. The method is particularly applicable tothe coating of metal substrates of relatively high thermal andelectrical conductivities, such as aluminum, copper, silver and thelike. In this connection, the invention is applicable to such metalsubstrates as steels, iron-base alloys, nickel and nickel-base alloys,cobalt and cobalt-base alloys and, generally, to those metals havingmelting points above I200F.

The invention is particularly applicable to those metal substrateshaving thermal and electrical conductivities of at least 0.2 referred tohigh electrical and thermal conductivity copper taken as 1. Thus, theinvention is applicable to the coating of the metals aluminum, copperand silver and Al-base, Cu-base and Agbase alloys. The metals copper andsilver and their alloys have particular use in electrical contacts wherewear resistance of the contacting face may be important.

Certain steel substrates can be further augmented in their properties byplasma spraying thereon a titanium carbide tool steel composition of thetype disclosed herein. One advantage of plasma spraying such substratesis in the building up of worn dies or dies inadver tently produced withan undersize.

A particular hard facing coating mentioned hcreinbefore is onecontaining to 80% by weight of TiC and the balance a steel matrixcontaining about 6 or 7% to [2% (r (preferably 8 to 12%), about 0.6 to1.2% C. about 0.5 to 50 Mo, up to about 5% W, up to about 2% V, up toabout 3% Ni, up to about 5% Co and the remainder essentially iron.

A specific coating composition is one containing about 40 to 50 byweight TiC, and the balance a steel matrix containing about l0% Cr,about l% C, about 3% Mo, about 1% V and the remainder essentially iron.Examples of two steel substrates coated with the foregoing type ofcomposition are as follows.

EXAMPLE 8 The coating is applicable to a steel substrate bearing theAlSl designation S2 (silicon tool steel) having the followingcomposition: 0.5 0.6% C, 0.4 0.6% Mn, 0.7 1.2% Si, 0. l5 0.3 V, 0.4 0.6Mo and the balance essentially iron. This steel exhibits good resistanceto shock. This property can be further augmented by providing a hardcoating on the surface thereof by plasma spraying the followingcomposition by weight: about 40% TiC dispersed through a steel matrixcontaining about l0% Cr, 1% C, 3% Mo. 1% V and the remainder essentiallyiron. The coating is deposited (about 0.015 inch thich) by plasmaspraying in accordance with the method of Example 4. After the coatinghas been smoothed by buffing, it is tempered at about 950/975F for about1 hour, cooled and then tempered again at the same temperature for lhour to in crease the hardness of the coating still further. Followingtempering, the coated substrate is ground to finished size. Generally,the tempering temperature may range from about 900F to [050F.

EXAMPLE 9 A worn die punch of a titanium carbide tool steel is built upby plasma spraying the surface thereof with a coating compositionsimilar to Example 8 containing by weight about 50% TiC dispersedthrough a steel matrix containing 10% Cr, l% C, 3% Mo and the balanceessentially iron. The composition of the substrate comprises 35% TiC byweight in a steel matrix containing 3% Cr, 3% M0, 0.5% C and the balanceessentially iron. The coating is deposited to a thickness of about 0.0linch, smoothed by buffing and then double tempered as in Example 8 at950/975F. Following tempering, the coated die punch is finished groundto the desired dimension. While the substrate is hard, the coatingprovides additional hardness and markedly improved wear resistance inlight of the higher titanium carbide content.

Thus, the invention provides as articles of manufacture composite metalstructures of ferrous and nonferrous metal substrates with adherentcoatings of titanium carbide steel compositions. Examples of suchcomposite metal structures produced by plasma spraying are as follows.

Cr, 2% Mo. 2% W. l /r C and the balance essentially iron As will beapparent, the metal substrate may be selected from the group consistingof steel and nonferrous metals. The non-ferrous metals of particular useas substrates are aluminum. copper and silver and alloys based on thesemetals, e.g., aluminum-base, copper-base and silver-base alloys,

As stated hereinbefore, the coating material should preferably bepre-alloyed before spraying so as to assure the presence of roundedgrains of primary titanium carbide dispersed through amartensiticcontaining steel matrix. The coatings in the foregoing tableby virtue of plasma spraying are generally characterized by the presenceof martensite in the matrix. If the matrix is too soft, thenpreferential wearing of the matrix will result in dislodgement oftitanium carbide and the gradual fretting away of the coating material.

Present developments in the rotary combustion engine contemplate the useof an aluminum housing. The rotating piston which has a generallytriangular shape is in contact with the end walls of the housing bymeans of the apices thereof which require the use ofa seal material as aseal-off between the spaces defined between the apices. The seal musthave wear resistance. However, the aluminum in the housing is generallysoft compared to most materials of construction and has poor wearresistance. By plasma spraying the inner walls of the aluminum housingwith a hard facing material of a titanium carbide tool steel compositionhaving low friction properties, the life of the housing can beprolonged. An aluminum housing is desirable as it is capable of beingair cooled easily in view of its high thermal conductivity. The heattreatable hard facing material should preferably be temper resistant soas to resist softening due to the generation of heat in the pistonduring fuel combustion.

A coating material which appears promising in that regard for thealuminum housing is one containing by weight about to 80% TiC and thebalance 90 to 20% of a steel matrix containing 6 or 7 to 12% Cr. 0.5 toM0, 0.6 to 1.2% C and the balance substantially iron. The matrixcomposition may contain up to about 5% W, up to about 3% Ni. up to about5% Co. up to about 2% V. amounts of Mn and Si usually found in steel andthe balance essentially iron. A specific composition is one containingCr, 3% Mo, l7r C and the balance essentially iron. This steel resiststempering at temperatures as high as l.000F and in fact increases inhardness due to a secondary hardening effect at the latter temperature.

HO. 2 shows schematically a rotary combustion engine comprising analuminum housing having a chamber 21 in which is mounted a triangularlyshaped rotary piston 22 in sealing contact with the end wall 23 of thechamber at its apices 24 to 26. The rotary piston has an internal gearmounted thereon which is driven by gear 28 mounted on a shaft runningperpendicular to the rotary piston. The hard facing material is appliedto end wall 23 as shown by the heavy line to provide sufficient wearresistance to the material of the apices in rubbing contact with the endwall. The material of the apices may comprise spring mounted inserts 29of the same titanium carbide tool steel maintained in continual sealingcontact with the end wall via spring 30.

In operation, as the piston rotates. fuel and air are received at intakezone 3! through intake 32. The fuel-air mixture is then compressed andfired in compression zone 33 via spark plug 34 and the combusted gasesat exhaust zone 35 exhausted through outlet 36. The temperature in thechamber rises to levels which may tend to temper certain heat treatablesteels. However. by employing a hard facing material describedhereinabove containing about 35% by weight of TiC dispersed through asteel matrix containing about 10% Cr, 3% Mo. 1% C and the balanceessentially iron, a temper resistant surface is provided capable ofbeing heated up to about l.000F without substantially diminishing inhardness and. if anything. increases in hardness due 0 a secondaryhardening effect engendered by the precipitation of secondary carbidescontaining chromium and/or through decomposition of retained austenite.By assuring the presence of rounded grains of titanium carbide in boththe inserts of the seal and the end wall hard facing material. lowfriction is assured during operation ofthe engine.

It is to be understood that the term "steel substrate" referred toherein also includes cast steels, cast irons and the iron-basematerials.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

What is claimed is:

1. A method of producing a hard, wear resistant coating of a heattreatable titanium carbide tool steel onto a metal substrate whichcomprises,

selecting a powder composition containing about ID to by weight ofprimary grains of titanium car bide and the balance essentially about to20% by weight of steel-forming ingredients which. when melted to form asteel and quenched from an austenitizing temperature. tends to formmartensite,

and then spraying said powder composition with a plasma flame directlyonto said metal substrate to quench-deposit a thin coating of saidcomposition thereon of thickness ranging up to about 0.025 inch.

thereby producing an adherent thin coating of said titanium carbide toolsteel composition on said metal substrate characterized by primarygrains of titanium carbide uniformly dispersed through a matrixcontaining an austenitic decomposition product containing martensite,and further characterized in that said coating is capable of being heattreated at a temperature below the melting point of the metal substrate,

2. The method of claim I, wherein the powder composition prior tospraying is a pre-alloyed composition characterized in that the titaniumcarbide grains in the particles thereof are rounded and dispersedthrough a steel matrix. such that the resulting adherent coating ischaracterized by the presence of said rounded grains of titaniumcarbide.

3. The method of claim 2, characterized in that the steel matrix isselected from the group consisting of(A) about 1 to 6% Cr, about 0.3 to6% Mo, about 0.3 to 0.8% C and the balance essentially iron; (B) about6% to l2% Cr, about 0.5 to Mo. about 0.6 to l.2 C. up to about 5% W. upto about 2% V. up to about 3% Ni. up to about 5% Co and the balanceessentially iron; and (C) a high nickel alloy steel containing about to30% Ni. about 0.2 to 9% Ti. up to about 5% Al, the sum of the Ti and Alcontent not exceeding about 9%. up to about 25% Co, up to about l0% Mo,substantially the balance of the matrix being at least about 50% iron,the metals making up the matrix composition being proportioned such thatwhen the nickel content ranges from about 10 to 22% and the sum of Aland Ti is less than about 1.5%, the molybdenum and cobalt contents areeach at least about 2%, and such that when the nickel content rangesfrom about 18 to 30% and the molybdenum content is less than 2%, the sumof Al and Ti exceeds l.5%; said matrix produced from compositions (A),(B) and (C) being characterized metallographically by the presence ofmartensite.

4. The method of claim 3, wherein the metal substrate is characterizedby a thermal and electrical conductivity relative to copper taken as lof at least about 0.2.

5. The method of claim 3, wherein the metal substrate is selected fromthe group consisting of steel and non-ferrous metals having a meltingpoint above l,l00F.

6. The method of claim 4, wherein said metal substrate is selected fromthe group consisting of aluminum, copper and silver and alloys based onsaid metals.

7. The method of claim 3, wherein the coating is derived fromcomposition (B) and wherein after the coated substrate has beenproduced, it is tempered at a temperature of about 900F to l,050F for atime at least sufficient to increase the hardness of the coating. a: a:

1. A METHOD OF PRODUCING A HARD, WEAR RESISTANT COATING OF A HEATTREATABLE TITANIUM CARBIDE TOOL STEEL ONTO A METAL SUBSTRATE WHICHCOMPRISES, SELECTING A POWDER COMPOSITION CONTAINING ABOUT 10 TO 80% BYWEIGHT OF PRIMARY GRAINS OF TITANIUM CARBIDE AND THE BALANCE ESSENTIALLYABOUT 90 TO 20% BY WEIGHT OF STEELFORMING INGREDIENTS WHICH, WHEN MELTEDTO FORM A STEEL AND QUENCHED FROM AN AUSTENITIZING TEMPERATURE, TENDS TOFORM MARTENSITE, AND THEN SPRAYING SAID POWDER COMPOSITION WITH A PLASMAFLAME DIRECTLY ONTO SAID METAL SUBSTRATE TO QUENCHDEPOSIT A THIN COATINGOF SAID COMPOSITION THEREON OF THICKNESS RANGING UP TO ABOUT 0.025 INCH,THEREBY PRODUCING AN ADHERENT THIN COATING OF SAID TITANIUM CARBIDE TOOLSTEEL COMPOSITION ON SAID METAL SUBSTRATE CHARACTERIZED BY PRIMARYGRAINS OF TITANIUM CARBIDE UNIFORMLY DISPERSED THROUGH A MATRIXCONTAINING AN AUSTENITIC DECOMPOSITION PRODUCT CONTAINING MARTENSITE,AND FURTHER CHARACTERIZED IN THAT SAID COATING IS CAPABLE OF BEING HEATTREATED AT A TEMPERATURE BELOW THE MELTING POINT OF THE METAL SUBSTRATE.2. The method of claim 1, wherein the powder composition prior tospraying is a pre-alloyed composition characterized in that the titaniumcarbide grains in the particles thereof are rounded and dispersedthrough a steel matrix, such that the resulting adherent coating ischaracterized by the presence of said rounded grains of titaniumcarbide.
 3. The method of claim 2, characterized in that the steelmatrix is selected from the group consisting of (A) about 1 to 6% Cr,about 0.3 to 6% Mo, about 0.3 to 0.8% C and the balance essentiallyiron; (B) about 6% to 12% Cr, about 0.5 to 5% Mo, about 0.6 to 1.2% C,up to about 5% W, up to about 2% V, up to about 3% Ni, up to about 5% Coand the balance esSentially iron; and (C) a high nickel alloy steelcontaining about 10 to 30% Ni, about 0.2 to 9% Ti, up to about 5% Al,the sum of the Ti and Al content not exceeding about 9%, up to about 25%Co, up to about 10% Mo, substantially the balance of the matrix being atleast about 50% iron, the metals making up the matrix composition beingproportioned such that when the nickel content ranges from about 10 to22% and the sum of Al and Ti is less than about 1.5%, the molybdenum andcobalt contents are each at least about 2%, and such that when thenickel content ranges from about 18 to 30% and the molybdenum content isless than 2%, the sum of Al and Ti exceeds 1.5%; said matrix producedfrom compositions (A), (B) and (C) being characterizedmetallographically by the presence of martensite.
 4. The method of claim3, wherein the metal substrate is characterized by a thermal andelectrical conductivity relative to copper taken as 1 of at least about0.2.
 5. The method of claim 3, wherein the metal substrate is selectedfrom the group consisting of steel and non-ferrous metals having amelting point above 1,100*F.
 6. The method of claim 4, wherein saidmetal substrate is selected from the group consisting of aluminum,copper and silver and alloys based on said metals.
 7. The method ofclaim 3, wherein the coating is derived from composition (B) and whereinafter the coated substrate has been produced, it is tempered at atemperature of about 900*F to 1, 050*F for a time at least sufficient toincrease the hardness of the coating.